The present invention relates to a technique for feedback controlling an air-fuel ratio of a combustion mixture in an internal combustion engine to a target air-fuel ratio.
It is common in an internal combustion engine mounted on a vehicle to feedback control an air-fuel ratio of a combustion mixture to a target value, in order to purify the exhaust gas or improve the fuel economy.
Therefore, a fuel supply amount is feedback controlled utilizing a PID control (proportional-plus-integral-plus-derivative control), while detecting sequentially an air-fuel ratio by an air-fuel ratio sensor disposed in an exhaust: passage and the like, so as to converge the detected air-fuel ratio to a target air-fuel ratio.
On the other hand, a sliding mode control is known for its high robust performance suppressing an influence by disturbances, which is often used to control robots. There has been a proposal to utilize the sliding mode control to the air-fuel ratio feedback control (refer to Japanese Unexamined Patent Publication No. 8-232713).
By the sliding mode control, the convergence performance is improved by swiftly guiding the system status onto a switching line (S=0). However, setting of the nonlinear term simply to a large value causes overshoot of the system status, which oscillates with great width centering on the switch line, and results in the fluctuation of the air-fuel ratio. In the above-mentioned sliding mode control, a deviation between a target air-fuel ratio and an actual air-fuel ratio is set as the switching function, and the nonlinear term is computed by integrating a feedback gain, the positive/negative of which is switched according to the positive/negative of the switching function. However, when the target air-fuel ratio is changed greatly, there is delay in the system status reaching the switching line, and a response characteristic is deteriorated. If the feedback gain is increased in order to suppress this delay, the nonlinear term to be integrated by the air-fuel ratio sensor during detection delay is increased, causing a large overshoot to increase a fluctuation of air-fuel ratio.
Even further, the fluctuation of air-fuel ratio is also caused in a system equipped with a fuel vapor processing device, which adsorbs and collects the fuel vapor generated in a fuel tank of a vehicle by a canister, and purges the fuel adsorbed and collected in the canister and supplies the fuel to an intake system (such as an intake collector) of the engine for combustion, under a predetermined operating condition.
Namely, a feedback control is performed so as to maintain the target air-fuel ratio by reducing a fuel injection amount to be injected by a fuel injection valve by an amount supplied to the engine by purging when fuel is purged from the canister. As a result, when the purging is cut off to suddenly stop the fuel supply from the canister to the engine, an air-fuel ratio fluctuation will be caused to shift the air-fuel ratio greatly to a lean side during a response delay of the feedback control.
The present invention aims at solving the problems of the prior art. The object of the present invention is to suppress a fluctuation of an air-fuel ratio, when an operating condition is switched so that the air-fuel ratio is changed during an air-fuel ratio feedback control.
Especially, the object of the present invention is to suppress the fluctuation of the air-fuel ratio, when a target air-fuel ratio is switched, or when the purging of fuel vapor is cut off in a case that an engine is equipped with a device for processing fuel vapor by purging the fuel vapor to an intake system of the engine.
Moreover, the object of the invention is to suppress the fluctuation of the air-fuel ratio easily by setting an appropriate nonlinear term when the air-fuel ratio is feedback controlled using a sliding mode control.
In order to achieve the above objects, the present invention is constituted to include:
computing an air-fuel ratio feedback control amount including a linear term and a nonlinear term by a sliding mode control.
feedback controlling an air-fuel ratio of a combustion mixture to a target air-fuel ratio, using the computed air-fuel ratio feedback control amount.
initializing the nonlinear term to a predetermined value to correspond to a post-switched operating condition when an operating condition where the air-fuel ratio is changed is switched.
According to this constitution, when the operating condition is switched so that the air-fuel ratio changes, the nonlinear term is initialized to a predetermined value set to correspond to the post-switched operating condition. Accordingly, the air-fuel ratio is swiftly converged to the air-fuel ratio corresponding to the switched operating condition, thereby enabling to suppress the fluctuation of the air-fuel ratio.
The time of switching of the operating condition is, for example, the time of when the target air-fuel ratio is switched, and therefore, the air-fuel ratio may be converged swiftly to the target air-fuel ratio.
Alternatively, in an engine equipped with a fuel vapor processing device for adsorbing and collecting fuel vapor generated in a fuel tank to a canister while supplying purged fuel from the canister to an intake system of the engine, the time of switching of the operating condition may be the time of when the purging is cut off. According to this constitution, the nonlinear term can be switched in stepwise to a predetermined value corresponding to the time of when the purging is cut off, thereby enabling to suppress the air-fuel ratio fluctuation during the purging cut off.
Further, the predetermined value may be stored in advance in a memory for each target air-fuel ratio. According to this constitution, the nonlinear term can be initialized to a predetermined value corresponding to the post-switched operating condition while saving the memory capacity.
Alternatively, the predetermined value may be computed in accordance with the target air-fuel ratio. Thereby, the air-fuel ratio is restrained to the vicinity of the target air-fuel ratio, to swiftly converge the air-fuel ratio to the target air-fuel ratio.
Even further, the linear term and the nonlinear term may be computed by a sliding mode control with a switching function thereof being a deviation between the air-fuel ratio and the target air-fuel ratio of the combustion mixture.
The setting of this switching function S is performed by a so-called direct switching function method of the sliding mode control, which defines, as the switching function S, a function realizing a state to be achieved by the switching plane (S=0) (in this case, the air-fuel ratio becoming the target air-fuel ratio). According to this method, the feedback control by the sliding mode control can be performed easily and with high accuracy.
Moreover, there is no need of a complex operation for modeling the engine when setting the switching function, and therefore, the present invention can be used generally to any engine without being influenced by the types of the vehicle or the engine.
Further, in the sliding mode control with the deviation mentioned above as the switching function, the nonlinear term may be computed by integrating a feedback gain, the positive/negative of which is switched in accordance with the positive/negative of the switching function.
According to this constitution, the positive/negative of the switching function is reversed every time the state of air-fuel ratio crosses the switching line, which causes the positive/negative of the feedback gain to be reversed and, by the nonlinear term obtained by integrating this feedback gain, the air-fuel ratio state can converged to the target air-fuel ratio while being restrained on the switching line.
Therefore, the air-fuel ratio can be converged to the target air-fuel ratio swiftly while being restrained in the vicinity of the target air-fuel ratio.
Moreover, an absolute value of the feedback gain may be set variably in accordance with the engine operating condition, such as, an engine load for example an intake air quantity or the rotation speed.
According to this constitution, the feedback gain whose absolute value is variably set in accordance with the engine operating condition can be used to compute the nonlinear term of the air-fuel ratio feedback control amount.
This enables to prevent the value of the nonlinear term from being integrated excessively during the detection delay time of the air-fuel ratio, and to perform a stable air-fuel ratio feedback control by reducing appropriately the deviation of the actual air-fuel ratio from the target air-fuel ratio, while maintaining the response characteristic.
Moreover, the linear term may be computed as a value proportional to a ratio of the deviation to the air-fuel ratio of the combustion mixture.
According to this constitution, the more the air-fuel ratio state deviates from the switching line, the greater the linear term is set in proportion to this deviation.
Thereby, the air-fuel ratio can be converged on the switching line toward the target air-fuel ratio swiftly, while suppressing overshoot.
The other objects and features of this invention will become understood from the following description with the accompanied drawings.